Optimization of core-shell nanoparticle layers for optical biosensing

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Abstract

In this work we constructed and optimized a label-free biosensor which is based on a combination of surface plasmon resonance and reflectometric interference. Both techniques have been utilized for label-free biosensing for more than two decades as the corresponding extinction spectra undergo a wavelength shift upon molecule binding. In the present study it has been demonstrated that a combination of both effects can significantly improve sensitivity. The developed biosensor consists of dielectric spheres of 400-500 nm diameters, deposited on a flat solid substrate and coated with gold nanoparticles. The spectrum of such structure exhibits multiple extinction peaks resulting from the interference of beams reflected between the flat substrate and the surface of the dielectric spheres. These peaks are enhanced by the presence of gold on top of the spheres due to coherent oscillation of the free electrons of the metal, i.e. plasmon excitation. In a systematic study, the optical properties of the sensing element have been optimized, and its sensitivity towards molecule binding has been tested by fibrinogen adsorption for the different sensor geometries developed. In the wavelength regime from 400-900 nm two dominant peaks are observed. It was shown, that the sensitivity of the peak between 600 and 900 nm exhibits the higher sensitivity compared to the peak between 400 and 600 nm. Different deposition techniques for the dielectric spheres have been tested to find the most reproducible one with closed packed coverage. Here, a technique, in which the dielectric spheres are first floated on a liquid subphase and then transferred to the solid support in a Langmuir-Blodgett like approach, yielded improved lateral homogeneity of the optical response and higher sensitivity than film of randomly deposited spheres. Two kinds of metallization have been studied (i) deposition of metal nanoparticles from solution (seeding) followed by an enlargement of the nanoparticles (plating), and (ii) evaporation of a metal thin film on the top of the spheres by physical vapor deposition (PVD). The resulting optical response and morphology were characterized by UV-Vis spectroscopy and scanning electron microscopy (SEM). For gold metal deposition from gold solution we found out that the sensitivity decreases with increasing plating time and is highest for purely seeded surfaces. For gold films deposited by PVD we identified an optimum gold thin film thickness of 50 nm to provide enhanced sensitivity. Effects of metal composition (gold and silver) on the optical properties and sensitivity have been investigated, showing significantly higher sensitivity for silver than for gold nanoparticle coatings of the same coverage in range from 400-600 nm. We also observed that the sensitivity is improved by the presence of a dielectric layer of silicon oxide/dioxide in between the substrate and the gold-coated spheres. Independent of the type of substrate used (i.e. metalized or not), an optimum layer thickness of 40 nm was found.